In the course of conversation, I put forward to a practitioner of medical Qigong that the experience of qi (chi) takes place in a state between waking and sleeping. He replied that he had once been in a lab hooked up to instruments while he practiced, and the folks watching the dials affirmed that he went through the first three stages of sleep with his eyes open. He agreed with me that this is the aspect of his practice that is difficult to teach, and said that his students get it through the practice of the art he teaches.
Olaf Blanke wrote:
In summary: A conflict between tactil/proprioceptive/kinesthetic and visual information coupled with a conflict between visual and vestibular information can, in some cases, give rise to a feeling that the self is in two places simultaneously, which can result in suicidal tendencies in the individual as they attempt to re-establish a unitary self at all cost.
(From a paper by Olaf Blanke and Christine Mohr, here)
Now I would suggest that the eyes are open in zazen for their influence in resetting the vestibular sense and providing a continuity in the sense of location, yet in zazen the proprioceptive sense must be allowed to influence the sense of location almost as though the eyes are closed. It’s a trick, a lot like falling asleep with your eyes open.